Wireless communication systems are advancing to provide good service quality, support a high data rate and keep up with the continuously increasing demand for wireless data traffic. The wireless communication systems, such as a Long Term Evolution (LTE) system, have been extended with support of device to device (D2D) features targeting both commercial and Public Safety applications. Some applications enabled by LTE are device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices. In 3GPP, all of these applications are defined under the umbrella of Proximity Services (ProSe).
One of the potential extensions of the ProSe framework consists of support of V2X communication, which includes any combination of direct communication between vehicles, pedestrians and infrastructure. The V2X communication may include V2V communication, V2P communication, V2I communication, and the like. In particular, the V2V is also referred to as vehicle-to-vehicle, which covers LTE-based communication between vehicles, the V2P is also referred to as vehicle-to-pedestrian, which covers LTE-based communication between a vehicle and a device carried by an individual (e.g. handheld terminal carried by a pedestrian, cyclist, driver or passenger), and the V2I is also referred to as vehicle-to-infrastructure which covers LTE-based communication between a vehicle and a roadside unit/network. V2X communication may take advantage of a NW infrastructure, when available, but at least basic V2X connectivity should be possible even in case of lack of coverage. Providing an LTE-based V2X interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the V2I, V2P and V2V communications.
From V2X perspective, a handover procedure of a terminal device may include a detach phase in which the terminal device detaches from its source cell and has not been connected to its target cell, and an abnormal phase in which the terminal device is connected to the target cell, but cannot use the resource in the target cell for V2X transmission. During the detach phase, the terminal device can neither obtain resource from the source cell nor the target cell for V2X transmission. At the same time, the terminal device is under network coverage, it cannot use pre-configured resources as a UE out of coverage can use. As a result, there will be service interruption during this phase, and the interruption time can be up to, for example, 2 seconds, which will be unacceptable for some V2X services.